Syrian President Bashar Al-Assad
Syrian President Bashar Al-AssadReuters

The Syrian government denied on Thursday that it used toxic gases in an attack in Aleppo earlier this week, after dozens of people had to be treated for breathing problems following a barrel bomb attack in Syria’s second largest city.

Following the attack, in the rebel-held Sukkari district of Aleppo city, the opposition accused the regime of dropping chlorine gas.

But in a statement carried by the official SANA state news agency on Thursday, the foreign ministry in Damascus denied the "use of toxic gases by any Syrian official side in Sukkari".

"The real criminal in all cases in which gases were used was armed terrorist groups who carry out the instructions of their masters in order to defame the achievements of the Syrian Arab Army," it charged.

“Terrorist groups” is the term used by President Bashar Al-Assad’s regime to describe all rebel groups fighting to oust him, including both Western-backed rebels and jihadist groups such as the Islamic State (ISIS).

On Wednesday, the Organization for the Prohibition of Chemical Weapons (OPCW) said it was "disturbed" by the alleged use of toxic chemicals in Aleppo.

"Such allegations are taken very seriously. The use of chemical weapons by anyone, anywhere and under any circumstances is unacceptable," said the director-general of the Hague-based OPCW, Ahmet Uzumcu.

The alleged chlorine attack on Aleppo came days after a UN-led investigation blamed the Syrian regime for at least two chemical attacks, one in 2014 and another in 2015.

Both sides in Syria's complex civil war have traded accusations of attacks against civilians and use of unconventional weapons including chlorine and mustard gas.

The government in Damascus argues that claims that it used chemical weapons are only meant to "serve political agendas".